"More than 12,000 Miles to the Gallon", Input/Output, ME Departments, January 2006

input/output

by Alan S. Brown, Associate Editor

More Than 12,000 Miles to the Gallon

The next time your friends brag about their hybrid car's fuel mileage, deflate their tires by mentioning the Shell Eco-Marathon's top-rated PAC-Car II. It set the world fuel economy record of 5,385 kilometers per liter (which sounds more impressive when converted to 12,666 miles per gallon).

The Eco-Marathon has hosted fuel economy races since 1985. This past year, it drew 227 high school and university teams to its preliminary race in Nogaro, France. They designed, built, and drove cars powered by conventional as well as solar, biofuel, and hydrogen fuel. The top 25 teams then ran 10 laps around the 1.3-mile Michelin test track in Ladoux, France, where PAC-Car II set the fuel economy record.

Driving the car is not for everyone. The three-wheeled vehicle is small, slow, and steers from behind. It has no doors and sits only half an inch above the ground, close enough to beach itself on even moderately sized potholes.

Nor does it really get 12,666 miles per gallon of gasoline. Its fuel cell runs on hydrogen, which is converted to its 95-octane lead-free gasoline equivalent to determine fuel economy.

At 64 pounds (not including the driver), the PAC car could be blown off its perch by a strong wind.

The fuel cell itself still needs work. Sitting behind an old Chevy in traffic probably exposes it to enough carbon monoxide to poison its catalyst. Fixing it means disassembling the entire fuel cell.

Still, 12,666 miles per gallon is an incredible achievement. ASME member Lino Guzzella, a professor of mechanical engineering at the Swiss Federal Institute of Technology (ETH) in Zurich who leads the project, said PAC-Car II showcases new technologies that may transform tomorrow's transportation.

Its carbon-reinforced epoxy shell, at 22 pounds, weighs about as much as a high school student's backpack. Yet it supports the entire load—including fuel cell, motors, axles, and occupant—without a separate chassis. Fully loaded, PAC-Car II weighs just 64 pounds, not including a necessarily petite student driver.

The car steers from its rear wheel. "It's like driving backwards all the time, but our drivers say you can get used to it," Guzzella said. The rudder-type design reduces aerodynamic drag up front. Because the front wheels don't steer, they can be squeezed together and enclosed in a sleek structure to slash frontal area to only 3 square feet. The tapered body yields a 0.075 drag coefficient, equivalent to an airplane wing and about one-fourth that of a Porsche Boxster.

Power comes from a 900-watt proton exchange membrane fuel cell. It weighs only 9 pounds, thanks to a new design that shaves nearly 12 pounds off the plates that apply 3.4 tons of pressure to the system. In the past, the team pressurized the fuel cell by sandwiching it between two 1.2-inch-thick aluminum plates fastened with long screws. A new design replaces the plates with lighter structures tensioned by glass fiber belts.

The fuel cell drives two servos attached to the rear wheel. One motor produces maximum torque; the other optimizes fuel efficiency. Both motors engage during full acceleration, but the torque motor cuts out once the car gets up to speed. The second motor can also disengage to let the car coast.

The ETH team plans carefully for each race. Using optimal control theory, they have discovered the best driving strategy. "We found there are only three possible optimal inputs: sustain speed, glide, and full acceleration," Guzzella said. "It's actually better to pull the throttle fully than partially, because motors convert electrical to mechanical energy more efficiently as they approach maximum load."

ETH also models each track's geometry, curves, and altitude, as well as potential wind factors and turbulence caused by other cars. They then download the strategy to an on-board computer that helps the driver stay on track.

The result is a car that gets 12,666 miles per gallon. You may not be able to buy one at your local car dealership, but with today's fuel prices, the goal it represents is far from academic.